Engine coolant leaking but no visible leak

Hi, my name is Devin. I am an auto repair expert with the Car Genius team here at RepairPal. I'm sorry to hear you're having trouble with your Passat. It sounds like you might be dealing with a leak that's difficult to spot. Since you've mentioned that coolant loss worsens when the engine is warmed up and at high RPMs, the coolant may be only leaking under high system pressure from somewhere not easily visible, like the radiator. Since this is a potentially severe issue, I recommend having a trusted mechanic inspect it thoroughly. If you need more advice, have any other questions about your vehicle, or would like to get connected to a RepairPal Certified shop near you, we have you covered. Give us a call at 877-913-1554, and we will happily help you out!